近日，美国加州大学圣巴巴拉分校的Chenhao Jin及其研究团队取得一项新进展。经过不懈努力，他们对魔角扭曲双层石墨烯的长寿命同位旋激发进行研究。相关研究成果已于2024年8月28日在国际权威学术期刊《自然》上发表。

本文将激子传感和光泵浦-探测光谱相结合，研究了整个平带范围内，以WSe₂为衬底的魔角扭曲双层石墨烯（MATBG）中的同位旋序动力学，实现了亚皮秒分辨率。研究人员观察到，在ν=2附近的宽填充范围内以及ν=-3和-2之间，同位旋动力学显著缓慢，其寿命长达300皮秒，与电子温度更快冷却（约10皮秒）的过程相分离。

这种非热行为表明同位旋自由度中存在异常长寿命的模式。这一未被理论预测到的观察结果意味着存在长程传播的集体模式、强烈的同位旋波动和记忆效应，并可能与区间相干或不相称的凯库勒螺旋基态有关。研究人员还进一步展示了先前在整数填充附近发现的同位旋序的非平衡控制。具体而言，通过超快操控，可以暂时使其偏离整数填充。本研究展示了MATBG中集体激发的独特探测方法，并为主动控制莫尔系统中的非平衡现象铺平了道路。

据悉，在魔角扭曲双层石墨烯（MATBG）中，已报道存在许多传统和奇异的相关多体相。然而，与这些相关状态相关的动力学，对于理解其背后的物理机制至关重要，却仍未被探索。

附：英文原文

Title: Long-lived isospin excitations in magic-angle twisted bilayer graphene

Author: Xie, Tian, Xu, Siyuan, Dong, Zhiyu, Cui, Zhiyuan, Ou, Yunbo, Erdi, Melike, Watanabe, Kenji, Taniguchi, Takashi, Tongay, Seth A., Levitov, Leonid S., Jin, Chenhao

Issue&Volume: 2024-08-28

Abstract: Numerous correlated many-body phases, both conventional and exotic, have been reported in magic-angle twisted bilayer graphene (MATBG). However, the dynamics associated with these correlated states, crucial for understanding the underlying physics, remain unexplored. Here we combine exciton sensing and optical pump–probe spectroscopy to investigate the dynamics of isospin orders in MATBG with WSe₂ substrate across the entire flat band, achieving sub-picosecond resolution. We observe remarkably slow isospin dynamics in a broad filling range around ν=2 and between ν=-3 and -2, with lifetimes of up to 300ps that decouple from the much faster cooling of electronic temperature (about 10ps). This non-thermal behaviour demonstrates the presence of abnormally long-lived modes in the isospin degrees of freedom. This observation, not anticipated by theory, implies the existence of long-range propagating collective modes, strong isospin fluctuations and memory effects and is probably associated with an intervalley coherent or incommensurate Kekulé spiral ground state. We further demonstrate non-equilibrium control of the isospin orders previously found around integer fillings. Specifically, through ultrafast manipulation, it can be transiently shifted away from integer fillings. Our study demonstrates a unique probe of collective excitations in MATBG and paves the way for actively controlling non-equilibrium phenomena in moiré systems.

DOI: 10.1038/s41586-024-07880-5

Source: https://www.nature.com/articles/s41586-024-07880-5